1. Field of the Invention
The present invention relates to a thermal infrared sensor and a manufacturing method thereof, and more particularly to a thermal infrared sensor and a manufacturing method thereof for receiving infrared rays from the back surface of a substrate.
2. Description of the Related Art
There is a technology for increasing the sensitivity of a thermal infrared sensor such as a thermal infrared image sensor. Specifically, a bridge structure is formed by removing part of the substrate around the infrared ray absorbing film by etching to thermally separate the infrared ray absorbing film and the substrate. There is known a thermal infrared sensor configured to receive infrared rays at the back surface (substrate side) of the infrared ray absorbing film that is supported in a thermally insulated manner.
For example, patent document 1 discloses a thermal infrared sensor in which an infrared ray absorbing film, a temperature sensor, and wiring used for reading signals are formed on a micro-bridge structure that is thermally separated from the substrate. This thermal infrared sensor is configured to receive infrared rays from the back surface of the substrate. Furthermore, the thermal infrared sensor includes antireflection films provided on both sides of the substrate, for the purpose of preventing reflection of the infrared rays entering from the back surface of the substrate.
FIG. 15 is a schematic cross-sectional view of an example of a conventional thermal infrared sensor.
A sensor part 101 supported by beam parts 102 is facing a substrate 103 across a hollow space 104. Thus, the sensor part 101 is thermally separated from the substrate 103, so that the sensitivity of the thermal infrared sensor is increased. The sensor part 101 includes an infrared ray absorbing film 105 and a temperature sensor 106. In this thermal infrared sensor, infrared rays enter from the back surface of the substrate 103, and the infrared ray absorbing film 105 is facing the substrate 103. This thermal infrared sensor detects infrared rays as the temperature sensor 106 captures temperature changes that occur when infrared rays entering from the substrate 103 are absorbed at the infrared ray absorbing film 105. In order to extract signals from the temperature sensor 106 from outside, wirings 107 are formed in the sensor part 101 and the beam parts 102.
Furthermore, there is a technology for increasing the sensitivity of a thermal infrared sensor that receives infrared rays at the front surface of a substrate. Specifically, an antireflection structure is formed by a sub wavelength structure to mitigate the reflection of infrared rays (see, for example, non-patent document 1).    Patent Document 1: Japanese Laid-Open Patent Publication No. H10-288550    Non-patent Document 1: 26th “Sensor micromachine and applied system” symposium paper collection, pp. 622-626, 2009
However, in a thermal infrared sensor that receives infrared rays from the back surface of a substrate, an antireflection structure or an antireflection film is not formed on the surface of the infrared ray absorbing film that is thermally separated from the substrate (the surface facing the substrate). Thus, some of the infrared rays entering from the back surface of the infrared ray absorbing film are reflected, and the sensor sensitivity is degraded.
For example, in the thermal infrared sensor illustrated in FIG. 15, when infrared rays enter from the back surface of the substrate 103, reflections occur at three positions, i.e., the back surface of the substrate 103, the interface between the substrate 103 and the hollow space 104, and the interface between the hollow space 104 and the infrared ray absorbing film 105. As to the back surface of the substrate 103, reflection can be prevented by forming an antireflection film. However, as to the interface between the substrate 103 and the hollow space 104 and the interface between the hollow space 104 and the infrared ray absorbing film 105, it is difficult to form an antireflection film, and therefore reflection cannot be prevented. In particular, as shown in the configuration of FIG. 15, when performing a method including a procedure of removing a part of the substrate 103 by etching to thermally separate the sensor part 101 from the substrate 103, the procedure of removing part of the substrate 103 by etching is performed toward the end of the method. Thus, antireflection films cannot be formed at the aforementioned interfaces in steps after the hollow space 104 has been formed.